Effects of global warming

The effects of global warming include the environmental, economic and social changes caused (directly or indirectly) by human emissions of greenhouse gases. There is a broad scientific consensus that climate change is occurring, and that human activities are the primary driver.[1] Many physical impacts of climate change have already been observed, including extreme weather events, glacier retreat,[2] changes in the timing of seasonal events[2] (e.g., earlier flowering of plants),[3] changes in agricultural productivity,[2] sea level rise, and declines in Arctic sea ice extent.[4] The potential impact of global warming depends on the extent to which nations implement prevention efforts and reduce greenhouse gas emissions. Ocean acidification is not a consequence of global warming, but instead has the same cause: increasing atmospheric carbon dioxide.

The social impact of climate changes will be further affected by our efforts to prepare for and adapt to changes that do occur.[5][6] Climate engineering is another policy option, although there are uncertainties regarding its effectiveness and little is known about potential side effects.[7] Climate change is likely to put pressure on some food crops and on fresh water supply. This in combination with extreme weather events, likely leads to negative effects on human health. Climate change can be a driver of human migration.

Near-term climate change policies could significantly affect long-term climate change impacts.[5][8] Stringent mitigation policies might be able to limit global warming (in 2100) to around 2 °C or below, relative to pre-industrial levels.[9] Without mitigation, increased energy demand and extensive use of fossil fuels[10] might lead to global warming of around 4 °C.[11][12] Higher magnitudes of global warming would be more difficult to adapt to,[13] and would increase the risk of negative impacts.[14]


Global warming refers to the long-term rise in the average temperature of the Earth's climate system. It is a major aspect of climate change, and has been demonstrated by the instrumental temperature record which shows global warming of around 1 °C since the pre-industrial period,[15] although the bulk of this (0.9°C) has occurred since 1970.[16] A wide variety of temperature proxies together prove that the 20th century was the hottest recorded in the last 2,000 years. Compared to climate variability in the past, current warming is also more globally coherent, affecting 98% of the planet.[17][18] The impact on the environment, eco-systems, the animal kingdom, society and humanity depends on how much more the Earth warms.[19]

The concept of global warming includes a key concern expressed by the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report which concluded, "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."[20] This has been brought about primarily through the burning of fossil fuels which has led to a significant increase in the concentration of GHGs in the atmosphere.[21] Records show that CO2 concentrations in the atmosphere rose from 325 ppm in 1972,[22] to over 400 ppm in 2015.[23] Atmospheric concentrations of carbon dioxide, methane and nitrous oxide are higher than they have been for at least the last 800,000 years.[24]

Global mean surface temperature change since 1880, relative to the 1951–1980 mean. Source: NASA GISS
The graph above shows the average of a set of temperature simulations for the 20th century (black line), followed by projected temperatures for the 21st century based on three greenhouse gas emissions scenarios (colored lines).[25]

Emission scenarios

Individual consumers, corporate decision makers, the fossil fuel industries, government responses and the extent to which different countries agree to cooperate all have a profound impact on how much greenhouse gases the worlds emits. As the crisis and modelling techniques have evolved, the IPCC and other climate scientists have tried a number of different tools to estimate likely greenhouse gas emissions in the future.

For instance, in 2000 the IPCC's Third Assessment Report included Special Report on Emissions Scenarios (SRES) which posed four possible future trajectories for greenhouse gases based on differing demographic, social, economic, technological, and environmental developments.[26] However, this approach omitted some significant changes to society and the global economy that were occurring, leading another group of researchers to develop the “Representative Concentration Pathways” (RCPs).[27]

RCP's were based on possible differences in radiative forcing occurring in the next 100 years but did not include socioeconomic “narratives” to go alongside them.[28] Another group of climate scientists, economists and energy system modellers took a different approach known as Shared Socioeconomic Pathways (SSPs); this is based on how socioeconomic factors such as population, economic growth, education, urbanisation and the rate of technological development might change over the next century. The SSPs describe five different trajectories which describe future climactic developments in the absence of new environmental policies beyond those in place today. They also explore the implications of different climate change mitigation scenarios.[29]

The range in temperature projections partly reflects the choice of emissions scenario, and the degree of "climate sensitivity".[30]:22–24 Different scenarios involve varying assumptions about future social and economic impact (e.g., economic growth, population level, energy policies), which in turn affects projections of greenhouse gas (GHG) emissions.[30]:22–24 The projected magnitude of warming by 2100 is closely related to the level of cumulative emissions over the 21st century (i.e. total emissions between 2000–2100).[31] The higher the cumulative emissions over this time period, the greater the level of warming is projected to occur.[31] Climate sensitivity reflects uncertainty in the response of the climate system to past and future GHG emissions.[30]:22–24 Higher estimates of climate sensitivity lead to greater projected warming, while lower estimates lead to less projected warming.[32]

Warming projections

The IPCC's Fifth Report, released in 2014, states that relative to the average from year 1850 to 1900, global surface temperature change by the end of the 21st century is likely to exceed 1.5 °C and may well exceed 2 °C for all RCP scenarios except RCP2.6. It is likely to exceed 2°C for RCP6.0 and RCP8.5, and more likely than not to exceed 2°C for RCP4.5. The IPCC says the pathway with the highest greenhouse gas emissions, RCP 8.5, will lead to a temperature increase of about 4.3˚C by 2100.[33] Warming will continue beyond 2100 under all RCP scenarios except RCP2.6.[34]

Breakthrough, an Australian report released in 2019 says current plans that countries put forward for cutting emissions in Paris may lead to around 3 °C of warming. Breakthrough says warming will likely be even higher than that because the model used does not include long-term carbon cycle feedback loops.[35] The Climate Action Tracker also says mitigation policies currently in place around the world will result in about 3.0°C warming above pre-industrial levels. However, if current plans are not actually implemented, global warming is expected to reach 4.1°C to 4.8°C by 2100.[36] Deploying worst-case scenario modelling where governments make no change to climate policy at all, a team of French scientists claim that global average temperatures could increase 7℃ by 2100.[37]

Even if emissions were drastically reduced overnight, the warming process is irreversible because CO
takes hundreds of years to break down, and global temperatures will remain close to their highest level for at least the next 1,000 years (see the later section on irreversibilities).[38][39][40]

Projected warming in historical context

Scientists have used various "proxy" data to assess past changes in Earth's climate (paleoclimate).[42] Sources of proxy data include historical records such as tree rings, ice cores, corals, and ocean and lake sediments.[42] The data suggest that recent warming has surpassed anything in the last 2,000 years.[43]

By the end of the 21st century, temperatures may increase to a level not experienced since the mid-Pliocene, around 3 million years ago.[44] At that time, models suggest that mean global temperatures were about 2–3 °C warmer than pre-industrial temperatures.[44] In the early Pliocene era, the global temperature was only 1-2 °C warmer than now, but sea level was 15–25 meters higher.[41][45]

Physical impacts

A broad range of evidence shows that the climate system has warmed.[48] Evidence of global warming is shown in the graphs (below right) from the US National Oceanic and Atmospheric Administration (NOAA). Some of the graphs show a positive trend, e.g., increasing temperature over land and the ocean, and sea level rise. Other graphs show a negative trend, such as decreased snow cover in the Northern Hemisphere, and declining Arctic sea ice, both of which are indicative of global warming. Evidence of warming is also apparent in living (biological) systems such as changes in distribution of flora and fauna towards the poles.[49]

Human-induced warming could lead to large-scale, abrupt and/or irreversible changes in physical systems.[50][51] An example of this is the melting of ice sheets, which contributes to sea level rise.[52] The probability of warming having unforeseen consequences increases with the rate, magnitude, and duration of climate change.[53]

Effects on weather

The main impact of global warming on the weather is an increase in extreme weather events such as heat waves, droughts, cyclones, blizzards and rainstorms. Of the 20 costliest climate and weather disasters that have occurred in the United States since 1980, eight have taken place since 2010, four of these in 2017 alone.[54] Such events will continue to occur more often and with greater intensity.[55] Episodes of intense precipitation contribute to flooding, soil erosion, landslides, and damage to structures and crops.[56]


Higher temperatures lead to increased evaporation and surface drying. As the air warms, its water-holding capacity also increases, particularly over the oceans. In general the air can hold about 7% more moisture for every 1C of temperature rise.[30] In the tropics, there’s more than a 10% increase in precipitation for a 1C increase in temperature.[59] Changes have already been observed in the amount, intensity, frequency, and type of precipitation. Extreme precipitation events are sometimes the result of atmospheric rivers - wide paths of atmospheric moisture composed of condensed water vapor.[60] Widespread increases in heavy precipitation have occurred even in places where total rain amounts have decreased.[61]

Projections of future changes in precipitation show overall increases in the global average, but with substantial shifts in where and how precipitation falls.[30]:24 Projections suggest a reduction in rainfall in the subtropics, and an increase in precipitation in subpolar latitudes and some equatorial regions.[58] In other words, regions which are dry at present will in general become even drier, while regions that are currently wet will in general become even wetter.[58] This projection does not apply to every locale, and in some cases can be modified by local conditions.[58] Although increased rainful will not occur everywhere, models suggest most of the world will have a 16-24% increase in heavy precipitation intensity by 2100.[62]


In the United States and many other parts of the world there has been a marked increase in the intense rainfall events which has resulted in more severe flooding. Minneapolis, for instance has had four 1000-year floods since the year 2000. In 2015, the floods were so high that people were literally fishing in the streets as lakes and streams overflowed and fish escaped the banks.[63]


Over most land areas since the 1950s, it is very likely that at all times of year both days and nights have become warmer[64][64] due to human activities.[64] There may have been changes in other climate extremes (e.g., floods, droughts and tropical cyclones) but these changes are more difficult to identify.[64] Projections suggest changes in the frequency and intensity of some extreme weather events.[64] In the U.S. since 1999, two warm weather records have been set or broken for every cold one.[65][66]

Some changes (e.g. more frequent hot days) will probably be evident in the near term (2016–2035), while other near-term changes (e.g. more intense droughts and tropical cyclones) are more uncertain.[64]

Future climate change will include more very hot days and fewer very cold days.[64] The frequency, length and intensity of heat waves will very likely increase over most land areas.[64] Higher growth in anthropogenic GHG emissions would cause more frequent and severe temperature extremes.[67] If GHG emissions grow a lot (IPCC scenario RCP8.5), already dry regions may have more droughts and less soil moisture.[68] Over most of the mid-latitude land masses and wet tropical regions, extreme precipitation events will very likely become more intense and frequent.[64]

Heat waves

Global warming boosts the probability of extreme weather events such as heat waves[69][70][71] where the daily maximum temperature exceeds the average maximum temperature by 5 °C (9 °F) for more than five consecutive days.[72]

In the last 30–40 years, heat waves with high humidity have become more frequent and severe. Extremely hot nights have doubled in frequency. The area in which extremely hot summers are observed has increased 50-100 fold. These changes are not explained by natural variability, and are attributed by climate scientists to the influence of anthropogenic climate change. Heat waves with high humidity pose a big risk to human health while heat waves with low humidity lead to dry conditions that increase wildfires. The mortality from extreme heat is larger than the mortality from hurricanes, lightning, tornadoes, floods, and earthquakes together.[73]


Prolonged periods of warmer temperatures typically cause soil and underbrush to be drier for longer periods, increasing the risk of wildfires. Hot, dry conditions increase the likelihood that wildfires will be more intense and burn for longer once they start.[74] Global warming has increased summertime air temperatures in California by over 3.5 degrees fahrenheit such that the fire season (the time before the winter rains dampen the vegetation) has lengthened by 75 days over previous decades. As a result, since the 1980s, both the size and ferocity of fires in California have increased dramatically. Since the 1970s, the size of the area burned has increased fivefold while fifteen of the 20 largest fires in California have occurred since 2000.[75]

In Australia, the annual number of hot days (above 35°C) and very hot days (above 40°C) has increased significantly in many areas of the country since 1950. The country has always had bushfires but in 2019, the extent and ferocity of these fires increased dramatically.[76] For the first time catastrophic bushfire conditions were declared for Greater Sydney. New South Wales and Queensland declared a state of emergency but fires were also burning in South Australia and Western Australia.[77]

Tropical cyclones

Although there will probably not be more tropical cyclones,[78] their wind speeds and rainfall will likely become more intense.[78] Changes in tropical cyclones will probably vary by region, but these variations are uncertain.[78]

Effects of climate extremes

The impacts of extreme events on the environment and human society will vary. Some impacts will be beneficial—e.g., fewer cold extremes will probably lead to fewer cold deaths.[79] Overall, however, impacts will probably be mostly negative.[80][81]


The cryosphere is made up of those parts of the planet which are so cold, they are frozen and covered by snow or ice. This includes ice and snow on land such as the continental ice sheets in Greenland and Antarctica, as well as glaciers and areas of snow and permafrost; and ice found on water including frozen parts of the ocean, such as the waters surrounding Antarctica and the Arctic.[82] The cryosphere, especially the polar regions, is extremely sensitive to changes in global climate.[83]

Arctic sea ice began to decline at the beginning of the twentieth century but the rate is accelerating. Since 1979, satellite records indicate the decline in summer sea ice coverage has been about 13% per decade.[84][85] The thickness of sea ice has also decreased by 66% or 2.0 m over the last six decades with a shift from permanent ice to largely seasonal ice cover.[86] As a result, some models project that Arctic sea ice in the summer could largely disappear by the end of the 21st century.[87] More recent projections suggest that the Arctic summers could be ice-free (defined as ice extent less than 1 million square km) as early as 2025–2030.[88]

Since the beginning of the twentieth century, there has also been a widespread retreat of alpine glaciers,[89] and snow cover in the Northern Hemisphere.[90] During the 21st century, glaciers[91] and snow cover are projected to continue their widespread retreat.[92] In the western mountains of North America, increasing temperatures and changes in precipitation are projected to lead to reduced snowpack.[93] Snowpack is the seasonal accumulation of slow-melting snow.[94] The melting of the Greenland and West Antarctic ice sheets could contribute to sea level rise, especially over long time-scales (see the section on Greenland and West Antarctic Ice sheets).[52]

Changes in the cryosphere are projected to have social impacts.[95] For example, in some regions, glacier retreat could increase the risk of reductions in seasonal water availability.[96] Barnett et al. (2005)[97] estimated that more than one-sixth of the world's population rely on glaciers and snowpack for their water supply.


Global warming is projected to have a number of effects on the oceans. Ongoing effects include rising sea levels due to thermal expansion and melting of glaciers and ice sheets, and warming of the ocean surface, leading to increased temperature stratification.[98] Other possible effects include large-scale changes in ocean circulation. The increase in ocean heat content is much larger than any other store of energy in the Earth's heat balance over the two periods 1961 to 2003 and 1993 to 2003, and accounts for more than 90% of the possible increase in heat content of the Earth system during these periods.[99] In 2019 a report published in the journal "Science" found the oceans are heating 40% faster than the IPCC predicted just five years ago.[100][101]

The oceans also serve as a sink for carbon dioxide, taking up much that would otherwise remain in the atmosphere, but increased levels of CO
have led to ocean acidification. Furthermore, as the temperature of the oceans increases, they become less able to absorb excess CO
. The oceans have also acted as a sink in absorbing extra heat from the atmosphere.[102]:4

Oxygen depletion

Warmer water cannot contain as much oxygen as cold water, so heating is expected to lead to less oxygen in the ocean. Other processes also play a role: stratification may lead to increases in respiration rates of organic matter, further decreasing oxygen content. The ocean has already lost oxygen, throughout the entire water column and oxygen minimum zones are expanding worldwide.[98] This has adverse consequences for ocean life.[103][104]

Sea level rise

The IPCC's Special Report on the Ocean and Cryosphere concluded that global mean sea level rose by 0.16 metres between 1901 and 2016.[106] The rate of sea level rise since the industrial revolution in the 19th century has been larger than the rate during the previous two thousand years (high confidence).[107]

Global sea level rise is accelerating, rising 2.5 times faster between 2006 and 2016 than it did during the 20th century.[108][109] Two main factors contribute to the rise. The first is thermal expansion: as ocean water warms, it expands. The second is from the melting of land-based ice in glaciers and ice sheets due to global warming.[110] Prior to 2007, thermal expansion was the largest component in these projections, contributing 70–75% of sea level rise.[111] As the impact of global warming has accelerated, melting from glaciers and ice sheets has become the main contributor.[112]

Even if emission of greenhouse gases stopped overnight, sea level rise will continue for centuries to come.[113] An assessment of the scientific literature on climate change, published in 2010 by the US National Research Council (US NRC, 2010),[114] described the IPCC projections as "conservative", and summarized the results of more recent studies which suggest a great deal of uncertainty in projections.[114] A range of projections suggest possible sea level rise by the end of the 21st century between 0.56 and 2 m, relative to sea levels at the end of the 20th century.[114]

In 2015, a study by Professor James Hansen of Columbia University and 16 other climate scientists said a sea level rise of three metres could be a reality by the end of the century.[115] Another study by scientists at the Royal Netherlands Meteorological Institute in 2017 using updated projections of Antarctic mass loss and a revised statistical method also concluded that, although it was a low probability, a three-metre rise was possible.[116] Seas expanding, due to the temperature rise and the melting of ice on Greenland and Antarctica, put at risk hundreds of millions of people in low lying coastal areas in countries such as China, Bangladesh, India and Vietnam.[117]

Ocean temperature rise

From 1961 to 2003, the global ocean temperature rose by 0.10 °C from the surface to a depth of 700 m. There is variability both year-to-year and over longer time scales, with global ocean heat content observations showing high rates of warming for 1991–2003, but some cooling from 2003 to 2007.[118] The temperature of the Antarctic Southern Ocean rose by 0.17 °C (0.31 °F) between the 1950s and the 1980s, nearly twice the rate for the world's oceans as a whole.[119] As well as having effects on ecosystems (e.g. by melting sea ice, affecting algae that grow on its underside), warming reduces the ocean's ability to absorb CO
. It is likely (greater than 66% probability, based on expert judgement)[120] that anthropogenic forcing contributed to the general warming observed in the upper several hundred metres of the ocean during the latter half of the 20th century.[121]

Regional effects

Temperatures across the world in the 1880s (left) and the 1980s (right), as compared to average temperatures from 1951 to 1980.[122]
Projected changes in average temperatures across the world in the 2050s under three greenhouse gas (GHG) emissions scenarios.[25]

Regional effects of global warming vary in nature. Some are the result of a generalised global change, such as rising temperature, resulting in local effects, such as melting ice. In other cases, a change may be related to a change in a particular ocean current or weather system. In such cases, the regional effect may be disproportionate and will not necessarily follow the global trend.

There are three major ways in which global warming will make changes to regional climate: melting or forming ice, changing the hydrological cycle (of evaporation and precipitation) and changing currents in the oceans and air flows in the atmosphere. The coast can also be considered a region, and will suffer severe impacts from sea level rise.

The Arctic, Africa, small islands and Asian megadeltas are regions that are likely to be especially affected by climate change.[123] Low-latitude, less-developed regions are at most risk of experiencing negative impacts due to climate change.[124] Developed countries are also vulnerable to climate change.[125] For example, developed countries will be negatively affected by increases in the severity and frequency of some extreme weather events, such as heat waves.[125] In all regions, some people can be particularly at risk from climate change, such as the poor, young children and the elderly.[123][124][126]

Projections of future climate changes at the regional scale do not hold as high a level of scientific confidence as projections made at the global scale.[127]:9 It is, however, expected that future warming will follow a similar geographical pattern to that seen already, with greatest warming over land and high northern latitudes, and least over the Southern Ocean and parts of the North Atlantic Ocean.[128] Nearly all land areas will very likely warm more than the global average.[129]

Social systems

Risk of climate-related impacts depends on physical hazards such as extreme weather or trends, whether people or ecosystems are present and exposed to these hazards and how vulnerable these groups are.[130] The vulnerability and exposure of human society to climate change varies. Sectors and industries at risk include freshwater quality and quantity, agriculture, human health, fisheries, forestry, energy, insurance, financial services, tourism, and recreation.[131] Rich countries, which have contributed most to the issue, are likely the least vulnerable to it.[132]

Food supply

Climate change will impact agriculture and food production around the world due to: the effects of elevated CO2 in the atmosphere, higher temperatures, altered precipitation and transpiration regimes, increased frequency of extreme events, and modified weed, pest, and pathogen pressure.[134] In general, low-latitude areas are at most risk of having decreased crop yields.[135]

As of 2007, the effects of regional climate change on agriculture have been small.[49] Changes in crop phenology provide important evidence of the response to recent regional climate change.[136] Phenology is the study of natural phenomena that recur periodically, and how these phenomena relate to climate and seasonal changes.[137] A significant advance in phenology has been observed for agriculture and forestry in large parts of the Northern Hemisphere.[49]


With low to medium confidence, Schneider et al. (2007)[19] projected that for about a 1 to 3 °C increase in global mean temperature (by the years 2090–2100, relative to average temperatures in the years 1990–2000), there would be productivity decreases for some cereals in low latitudes, and productivity increases in high latitudes. With medium confidence, global production potential was projected to:[19]

  • increase up to around 3 °C,
  • very likely decrease above about 3 °C.

Most of the studies on global agriculture assessed by Schneider et al. (2007)[135] had not incorporated a number of critical factors, including changes in extreme events, or the spread of pests and diseases. Studies had also not considered the development of specific practices or technologies to aid adaptation to climate change.[135]

The graphs opposite show the projected effects of climate change on selected crop yields.[139] Actual changes in yields may be above or below these central estimates.[139]

The projections above can be expressed relative to pre-industrial (1750) temperatures.[140] 0.6 °C of warming is estimated to have occurred between 1750 and 1990–2000. Add 0.6 °C to the above projections to convert them from a 1990–2000 to pre-industrial baseline.

Food security

Easterling et al. (2007)[141] assessed studies that made quantitative projections of climate change impacts on food security. It was noted that these projections were highly uncertain and had limitations. However, the assessed studies suggested a number of fairly robust findings. The first was that climate change would likely increase the number of people at risk of hunger compared with reference scenarios with no climate change. Climate change impacts depended strongly on projected future social and economic development. Additionally, the magnitude of climate change impacts was projected to be smaller compared to the impact of social and economic development. In 2006, the global estimate for the number of people undernourished was 820 million.[142] Under the SRES A1, B1, and B2 scenarios (see the SRES article for information on each scenario group), projections for the year 2080 showed a reduction in the number of people undernourished of about 560–700 million people, with a global total of undernourished people of 100–240 million in 2080. By contrast, the SRES A2 scenario showed only a small decrease in the risk of hunger from 2006 levels. The smaller reduction under A2 was attributed to the higher projected future population level in this scenario.

Droughts and agriculture

Some evidence suggests that droughts have been occurring more frequently because of global warming and they are expected to become more frequent and intense in Africa, southern Europe, the Middle East, most of the Americas, Australia, and Southeast Asia.[143] However, other research suggests that there has been little change in drought over the past 60 years.[144] Their impacts are aggravated because of increased water demand, population growth, urban expansion, and environmental protection efforts in many areas.[145] Droughts result in crop failures and the loss of pasture grazing land for livestock.[146]


Human beings are exposed to climate change through changing weather patterns (temperature, precipitation, sea-level rise and more frequent extreme events) and indirectly through changes in water, air and food quality and changes in ecosystems, agriculture, industry and settlements and the economy (Confalonieri et al., 2007:393).[79]

A study by the World Health Organization (WHO, 2009)[147] estimated the effect of climate change on human health. Not all of the effects of climate change were included in their estimates, for example, the effects of more frequent and extreme storms were excluded. Climate change was estimated to have been responsible for 3% of diarrhoea, 3% of malaria, and 3.8% of dengue fever deaths worldwide in 2004. Total attributable mortality was about 0.2% of deaths in 2004; of these, 85% were child deaths. The air pollution, wildfires, heat waves caused by the effects of global warming have significantly affected human health.[148]


With high confidence, authors of the IPCC AR4 Synthesis report[149]:48 projected that climate change would bring some benefits in temperate areas, such as fewer deaths from cold exposure, and some mixed effects such as changes in range and transmission potential of malaria in Africa. Benefits were projected to be outweighed by negative health effects of rising temperatures, especially in developing countries.

With very high confidence, Confalonieri et al. (2007)[79]:393 concluded that economic development was an important component of possible adaptation to climate change. Economic growth on its own, however, was not judged to be sufficient to insulate the world's population from disease and injury due to climate change. Future vulnerability to climate change will depend not only on the extent of social and economic change, but also on how the benefits and costs of change are distributed in society.[150] For example, in the 19th century, rapid urbanization in western Europe lead to a plummeting in population health.[150] Other factors important in determining the health of populations include education, the availability of health services, and public-health infrastructure.[79]:393

On mental health

In 2018, the American Psychological Association issued a report about the impact of climate change on mental health. It said that "gradual, long-term changes in climate can also surface a number of different emotions, including fear, anger, feelings of powerlessness, or exhaustion".[151]

Water resources

A number of climate-related trends have been observed that affect water resources. These include changes in precipitation, the crysosphere and surface waters (e.g., changes in river flows).[152] Observed and projected impacts of climate change on freshwater systems and their management are mainly due to changes in temperature, sea level and precipitation variability.[153] Changes in temperature are correlated with variability in precipitation because the water cycle is reactive to temperature.[154] The shift in temperature is mostly caused by human fossil fuel use in the 20th century.[155] According to NASA's statistics the global temperature increase has risen 1.4 degrees Fahrenheit since 1975.[155] The small but significant temperature increase creates a domino effect of issues because it begins with a shift in precipitation patterns. Excessive precipitation patterns lead to excessive sediment deposition, nutrient pollution, and concentration of minerals in aquifers. The rising global temperature will cause sea level rise and will extend areas of salinization of groundwater and estuaries, resulting in a decrease in freshwater availability for humans and ecosystems in coastal areas. The exposure of rising sea level will push the salt gradient into freshwater deposits and will eventually pollute freshwater sources. In an assessment of the scientific literature, Kundzewicz et al. (2007)[153] concluded, with high confidence, that:

Technological Freshwater Uses:

Freshwater has become an aiding factor for industrialization in this modern era. It has many uses other than drinking including: domestic use, irrigation, livestock, aquaculture, industrial, mining, public supply, and thermoelectric. These are only some of the general uses of freshwater that further complicate freshwater quality. These components take a large quantity of freshwater to implement into technology. For a reference, measured in million gallons per day, public use accounts for roughly 44,000 mg/pd, Domestic use 4,000 mg/pd, Irrigation 128,000 mg/pd, livestock 2,140 mg/pd, aquaculture 8,780 mg/pd, industrial 17,000 mg/pd, mining 2,310 mg/pd, and finally thermoelectric 143,000 mg/pd. The amount of freshwater being allocated towards technology results for about half of the natural freshwater resource that is actually available to us. With all of these different factors using the freshwater resource that accounts for less than one percent it should be of concern. Current water energy regulations are being made to switch to less energy intensive processes. In turn, lowering water use has a direct link with energy use, significantly lowering amount of emissions. reason being that there is a water-energy nexus and that they work in synergy. Water is needed to produce energy while energy is needed to "produce" water.[159] Examining this relationship can significantly lower greenhouse emissions, resulting in slower rates of climate change.

Migration and conflict

General circulation models project that the future climate change will bring wetter coasts, drier mid-continent areas, and further sea level rise.[160] Such changes could result in the gravest effects of climate change through human migration.[161] Millions might be displaced by shoreline erosions, river and coastal flooding, or severe drought.

Migration related to climate change is likely to be predominantly from rural areas in developing countries to towns and cities.[160]:407[162] In the short term climate stress is likely to add incrementally to existing migration patterns rather than generating entirely new flows of people.[162]:110

It has been argued that environmental degradation, loss of access to resources (e.g., water resources),[163] and resulting human migration could become a source of political and even military conflict.[164] Factors other than climate change may, however, be more important in affecting conflict. For example, Wilbanks et al. (2007)[165] suggested that major environmentally influenced conflicts in Africa were more to do with the relative abundance of resources, e.g., oil and diamonds, than with resource scarcity. Scott et al. (2001) placed only low confidence in predictions of increased conflict due to climate change.[164]

A 2013 study found that significant climatic changes were associated with a higher risk of conflict worldwide, and predicted that "amplified rates of human conflict could represent a large and critical social impact of anthropogenic climate change in both low- and high-income countries."[166] Similarly, a 2014 study found that higher temperatures were associated with a greater likelihood of violent crime, and predicted that global warming would cause millions of such crimes in the United States alone during the 21st century.[167] A 2018 study in the journal Nature Climate Change found that previous studies on the relationship between climate change and conflict suffered from sampling bias and other methodological problems.[168]

Military planners are concerned that global warming is a "threat multiplier". "Whether it is poverty, food and water scarcity, diseases, economic instability, or threat of natural disasters, the broad range of changing climatic conditions may be far reaching. These challenges may threaten stability in much of the world".[169] For example, the onset of Arab Spring in December 2010 is partly the result of a spike in wheat prices following crop losses from the 2010 Russian heat wave.[170][171]

Aggregate impacts

Aggregating impacts adds up the total impact of climate change across sectors and/or regions.[172] Examples of aggregate measures include economic cost (e.g., changes in gross domestic product (GDP) and the social cost of carbon), changes in ecosystems (e.g., changes over land area from one type of vegetation to another),[173] human health impacts, and the number of people affected by climate change.[174] Aggregate measures such as economic cost require researchers to make value judgements over the importance of impacts occurring in different regions and at different times.

Observed impacts

Global losses reveal rapidly rising costs due to extreme weather-related events since the 1970s.[175] Socio-economic factors have contributed to the observed trend of global losses, e.g., population growth, increased wealth.[176] Part of the growth is also related to regional climatic factors, e.g., changes in precipitation and flooding events. It is difficult to quantify the relative impact of socio-economic factors and climate change on the observed trend.[176] The trend does, however, suggest increasing vulnerability of social systems to climate change.[176][177]

Projected impacts

The total economic impacts from climate change are highly uncertain.[178] With medium confidence, Smith et al. (2001)[179] concluded that world GDP would change by plus or minus a few percent for a small increase in global mean temperature (up to around 2 °C relative to the 1990 temperature level). Most studies assessed by Smith et al. (2001)[179] projected losses in world GDP for a medium increase in global mean temperature (above 2–3 °C relative to the 1990 temperature level), with increasing losses for greater temperature increases. This assessment is consistent with the findings of more recent studies, as reviewed by Hitz and Smith (2004).[180]

Economic impacts are expected to vary regionally.[180][181][182] For a medium increase in global mean temperature (2–3 °C of warming, relative to the average temperature between 1990–2000), market sectors in low-latitude and less-developed areas might experience net costs due to climate change.[19] On the other hand, market sectors in high-latitude and developed regions might experience net benefits for this level of warming. A global mean temperature increase above about 2–3 °C (relative to 1990–2000) would very likely result in market sectors across all regions experiencing either declines in net benefits or rises in net costs.[52]

In 2019 the National Bureau of Economic Research found that increase in average global temperature by 0.04 °C per year, in absence of mitigation policies, will reduces world real GDP per capita by 7.22% by 2100. Following the Paris Agreement, thereby limiting the temperature increase to 0.01 °C per year, reduces the loss to 1.07%[183][184]

Aggregate impacts have also been quantified in non-economic terms. For example, climate change over the 21st century is likely to adversely affect hundreds of millions of people through increased coastal flooding, reductions in water supplies, increased malnutrition and increased health impacts.[81]

Sense of crisis

In 2018, Breakthrough released a report describing a climate change doomsday scenario by 2050 if we don't act soon. It said “feedback cycles could push warming to 3C by 2050, making climate change a near- to mid-term existential threat to human civilization”. It went on to say that "irreversible damage" is happening to global climate systems which may result "in a world of chaos where political panic is the norm and we are on a path facing the end of civilisation".[185][186] Commenting on the report, Adam Sobel professor of applied physics & mathematics at Columbia University said: "Three degrees Celsius by 2100 is a pretty middle-of-the-road estimate. It's not extreme and it's totally believable if serious action isn't taken."[187]

In response to the threat posed by global warming, in 2019 some media outlets began using the term climate crisis instead of climate change[188] while a few countries declared a climate emergency.[189] Joseph Stiglitz, Nobel laureate in economics, Professor at Columbia University, and former chief economist of the World Bank says: “The climate emergency is our third world war. Our lives and civilization as we know it are at stake, just as they were in the Second World War.” [190]

Biological systems

Observed impacts on biological systems

With very high confidence, Rosenzweig et al. (2007) concluded that recent warming had strongly affected natural biological systems.[49] Hundreds of studies have documented responses of ecosystems, plants, and animals to the climate changes that have already occurred.[192] For example, in the Northern Hemisphere, species are almost uniformly moving their ranges northward and up in elevation in search of cooler temperatures.[193] Humans are very likely causing changes in regional temperatures to which plants and animals are responding.[193]

Projected impacts on biological systems

By the year 2100, ecosystems will be exposed to atmospheric CO
levels substantially higher than in the past 650,000 years, and global temperatures at least among the highest of those experienced in the past 740,000 years.[194] Significant disruptions of ecosystems are projected to increase with future climate change.[195] Examples of disruptions include disturbances such as fire, drought, pest infestation, invasion of species, storms, and coral bleaching events. The stresses caused by climate change, added to other stresses on ecological systems (e.g., land conversion, land degradation, harvesting, and pollution), threaten substantial damage to or complete loss of some unique ecosystems, and extinction of some critically endangered species.[195][196]

Climate change has been estimated to be a major driver of biodiversity loss in cool conifer forests, savannas, mediterranean-climate systems, tropical forests, in the Arctic tundra, and in coral reefs.[197] In other ecosystems, land-use change may be a stronger driver of biodiversity loss at least in the near-term.[197] Beyond the year 2050, climate change may be the major driver for biodiversity loss globally.[197]

A literature assessment by Fischlin et al. (2007)[194] included a quantitative estimate of the number of species at increased risk of extinction due to climate change. With medium confidence, it was projected that approximately 20 to 30% of plant and animal species assessed so far (in an unbiased sample) would likely be at increasingly high risk of extinction should global mean temperatures exceed a warming of 2 to 3 °C above pre-industrial temperature levels.[194] The uncertainties in this estimate, however, are large: for a rise of about 2 °C the percentage may be as low as 10%, or for about 3 °C, as high as 40%, and depending on biota (all living organisms of an area, the flora and fauna considered as a unit)[198] the range is between 1% and 80%.[197] As global average temperature exceeds 4 °C above pre-industrial levels, model projections suggested that there could be significant extinctions (40–70% of species that were assessed) around the globe.[197]

Assessing whether future changes in ecosystems will be beneficial or detrimental is largely based on how ecosystems are valued by human society.[199] For increases in global average temperature exceeding 1.5 to 2.5 °C (relative to global temperatures over the years 1980–1999)[200] and in concomitant atmospheric CO
concentrations, projected changes in ecosystems will have predominantly negative consequences for biodiversity and ecosystems goods and services, e.g., water and food supply.[201]

Abrupt or irreversible changes

Physical, ecological and social systems may respond in an abrupt, non-linear or irregular way to climate change.[202] This is as opposed to a smooth or regular response. A quantitative entity behaves "irregularly" when its dynamics are discontinuous (i.e., not smooth), nondifferentiable, unbounded, wildly varying, or otherwise ill-defined.[202] Such behaviour is often termed "singular". Irregular behaviour in Earth systems may give rise to certain thresholds, which, when crossed, may lead to a large change in the system.

Some singularities could potentially lead to severe impacts at regional or global scales.[203] Examples of "large-scale" singularities are discussed in the articles on abrupt climate change, climate change feedback and runaway climate change. It is possible that human-induced climate change could trigger large-scale singularities, but the probabilities of triggering such events are, for the most part,[204] poorly understood.[203]

With low to medium confidence, Smith et al. (2001)[202] concluded that a rapid warming of more than 3 °C above 1990 levels would exceed thresholds that would lead to large-scale discontinuities in the climate system. Since the assessment by Smith et al. (2001), improved scientific understanding provides more guidance for two large-scale singularities: the role of carbon cycle feedbacks in future climate change and the melting of the Greenland and West Antarctic ice sheets.[180]

A 2018 study states that 45% of the environmental problems, including those caused by climate change are interconnected and make the risk of "domino effect" bigger.[205][206]

Biogeochemical cycles

Climate change may have an effect on the carbon cycle in an interactive "feedback" process. A feedback exists where an initial process triggers changes in a second process that in turn influences the initial process. A positive feedback intensifies the original process, and a negative feedback reduces it.[149]:78 Models suggest that the interaction of the climate system and the carbon cycle is one where the feedback effect is positive.[207]:792

Using the A2 SRES emissions scenario, Schneider et al. (2007)[207]:789 found that this effect led to additional warming by the years 2090–2100 (relative to the 1990–2000) of 0.1–1.5 °C. This estimate was made with high confidence. The climate projections made in the IPCC Fourth Assessment Report summarized earlier of 1.1–6.4 °C account for this feedback effect. On the other hand, with medium confidence, Schneider et al. (2007)[207]:789 commented that additional releases of GHGs were possible from permafrost, peat lands, wetlands, and large stores of marine hydrates at high latitudes.

Greenland and West Antarctic Ice sheets

With medium confidence, authors of AR4[52] concluded that with a global average temperature increase of 1–4 °C (relative to temperatures over the years 1990–2000), at least a partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic ice sheets would occur. The estimated timescale for partial deglaciation was centuries to millennia, and would contribute 4 to 6 metres (13 to 20 ft) or more to sea level rise over this period.[208]

Atlantic Meridional Overturning Circulation

The Atlantic Meridional Overturning Circulation (AMOC) is an important component of the Earth's climate system, characterized by a northward flow of warm, salty water in the upper layers of the Atlantic and a southward flow of colder water in the deep Atlantic.[210]:5 The AMOC is equivalently known as the thermohaline circulation (THC).[180] Potential impacts associated with MOC changes include reduced warming or (in the case of abrupt change) absolute cooling of northern high-latitude areas near Greenland and north-western Europe, an increased warming of Southern Hemisphere high-latitudes, tropical drying, as well as changes to marine ecosystems, terrestrial vegetation, oceanic CO
uptake, oceanic oxygen concentrations, and shifts in fisheries.[211] According to an assessment by the US Climate Change Science Program (CCSP, 2008b),[210]:5 it is very likely (greater than 90% probability, based on expert judgement)[210]:12 that the strength of the AMOC will decrease over the course of the 21st century. Warming is still expected to occur over most of the European region downstream of the North Atlantic Current in response to increasing GHGs, as well as over North America. Although it is very unlikely (less than 10% probability, based on expert judgement)[210]:12 that the AMOC will collapse in the 21st century, the potential consequences of such a collapse could be severe.[210]:5


Commitment to radiative forcing

Emissions of GHGs are a potentially irreversible commitment to sustained radiative forcing in the future.[212] The contribution of a GHG to radiative forcing depends on the gas's ability to trap infrared (heat) radiation, the concentration of the gas in the atmosphere, and the length of time the gas resides in the atmosphere.[212]

is the most important anthropogenic GHG.[213] While more than half of the CO
emitted is currently removed from the atmosphere within a century, some fraction (about 20%) of emitted CO
remains in the atmosphere for many thousands of years.[214] Consequently, CO
emitted today is potentially an irreversible commitment to sustained radiative forcing over thousands of years.

This commitment may not be truly irreversible should techniques be developed to remove CO
or other GHGs directly from the atmosphere, or to block sunlight to induce cooling.[38] Techniques of this sort are referred to as geoengineering. Little is known about the effectiveness, costs or potential side-effects of geoengineering options.[215] Some geoengineering options, such as blocking sunlight, would not prevent further ocean acidification.[215]

Irreversible impacts

Human-induced climate change may lead to irreversible impacts on physical, biological, and social systems.[216] There are a number of examples of climate change impacts that may be irreversible, at least over the timescale of many human generations.[217] These include the large-scale singularities described above – changes in carbon cycle feedbacks, the melting of the Greenland and West Antarctic ice sheets, and changes to the AMOC.[217] In biological systems, the extinction of species would be an irreversible impact.[217] In social systems, unique cultures may be lost due to climate change.[217] For example, humans living on atoll islands face risks due to sea-level rise, sea-surface warming, and increased frequency and intensity of extreme weather events.[218]

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Further reading

Physical impacts
Social, economic and ecological impacts
  • Climate change on the United Nations Economic and Social Development (UNESD) Division for Sustainable Development website.
  • The IPCC Working Group II (WG II) website at the Wayback Machine (archived 2010-02-25) – This body assesses the vulnerability of socio-economic and natural systems to climate change, negative and positive consequences of climate change, and options for adapting to it.
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